Are you using Codex?

I'm not sure how big your repos are but I've been effective working with repos that have thousands of files and tens of thousands of lines of code.

If you're just prototyping it will hit wall when things get unwieldy but that's normally a sign that you need to refactor a bit.

Super strict compiler settings, static analysis, comprehensive tests, and documentation help a lot. As does basic technical design. After a big feature is shipped I do a refactor cycle with the LLM where we do a comprehensive code review and patch things up. This does require human oversight because the LLMs are still lacking judgement on what makes for good code design.

The places where I've seen them be useless is working across repositories or interfacing with things like infrastructure.

It's also very model-dependent. Opus is a good daily driver but Codex is much better are writing tests for some reason. I'll often also switch to it for hard problems that Claude can't solve. Gemini is nice for 'I need a prototype in the next 10 minutes', especially for making quick and dirty bespoke front-ends where you don't care about the design just the functionality.

> tens of thousands of lines of code

Perhaps this is part of it? Tens of thousands of lines of code seems like a very small repo to me.